1. Technical Field of the Invention
The invention relates to a method for solving optimisation problems occurring during the design of a mostly optical ATM network.
2. Description of Related Art
The technique of finding new solutions for designing networks based on Asynchronous Transfer Mode (ATM) or on All Optical Networks (AON) is a rapidly growing area of research in telecommunication and computer business. Both ATM and AON have own methodologies for simplifying network management and switching. Whereas ATM networks apply the VP (Virtual Path) concept, i.e many connections which have a common path in a part of the network are handled (managed, switched) together, the AONs assign the wavelengths to connections in such a way that connections of the same wavelength are handled together without any electrical processing during the transmission. Optical networks employ Wavelength Division Multiplexing (WDM) and specially designed algorithms allow a high aggregate system capacity due to spatial reuse of wavelengths.
A known idea, (Mukherjee, Ramamurthy and Banerjee, “Some principles for designing a wide-area optical network” IEEE 1994, pp. 110-119), is to combine the technology of AONs with ATM technique. By using optical amplifiers and switches in a mostly optical ATM network, the number of opto-electrical conversions can be reduced significantely, which leads to better signal quality and lower delays, but from the point of view of ATM there is a problem: the VPI/NCI modification in the cell header becomes impossible. The Wavelength Division Multiplexing (WDM) technique offers a good utilization of the fibre but the transmission capacity is divided into equal parts (channels) defined by the wavelengths used. Because of this partitioned capacity the statistical multiplexing (SM), which is another benefit of the ATM, has limits. SM can shortly be described as the gain accomplished when dynamically multiplexing together channels with bursty traffic into one channel, thereby eliminating those empty timeslots which may occur in the channels due to the burstiness. The larger the capacity, the lower the effective bandwith, therefore the resource usage is more efficient.
Combining the technology of AONs with ATM technique entails a loss of performance due to the loss of SM. The total effect though, is a gain due to the enhanced performance coming from the simplified VP management.
A use of this combined technique implies both advantages and drawbacks. The drawbacks are that the switching must be done on three different levels (instead of two), namely: 1. Pure optical switching for VPLs; 2. VP switching (Cross connects) where the switching is done on the basis of VPIs (VPIs are changed); 3. VC switching. The design procedure is computationally hard as the complexity is about the number of available wavelengths to the power of the number of nodes in the network. The main advantage though, is that many opto-electrical conversions can be avoided so that less CCs are required, thereby enhancing the performance.
There is a problem in how to make the architecture of systems like this optimal, i.e when designing backbone networks for example, one must assume full logical connectivity between the nodes in the network and try to find optimisation algorithms for best using the limited number of wavelengths to accomplish a high aggregate system capacity due to spatial reuse of wavelengths.
The architecture used in the IEEE-paper is a combination of well-known “singlehop” and “multihop” approaches as employed in many other WDM networks/proposals. The paper deals with optimising the cost, efficiency and throughput of the routing node by theorethical analysis. The optimisation is here split into two steps with the objective of minimising the average message delay caused by transfer: 1. They find and map a virtual topology into the given physical; and 2. Assign the wavelengths to links of the virtual topology. This technique brings about that the VP design is already performed in step one, meaning that the optimality criteria could be deteriorated and the delays could increase since if there are more end-to-end streams on a physical link than the allowed number of wavelengths, the paper proposes a rerouting of one of these streams. Hence, the average delay is increased as the wavelength assignment and the virtual topology forming are divided.